Image display apparatus

ABSTRACT

An image display device according to the present invention is capable of presenting to a viewer a high-quality lustrous video on a display screen with best screen brightness by increasing visual contrast and avoiding loss of true black elements without widening a dynamic range of a picture signal. The image display device is provided with a liquid crystal display portion ( 11 ), a display control portion ( 14 ), a backlight ( 12 ), a backlight control portion ( 13 ) and an average brightness detecting portion ( 15 ) and detects brightness of the light source in accordance with the average brightness of a picture signal to be displayed. It is further provided with a peak detecting portion ( 16 ) and corrects the control of the backlight control portion ( 13 ) in accordance with a detected peak value of a picture.

FIELD OF THE INVENTION

[0001] The present invention relates to a liquid crystal type imagedisplay device for displaying a picture signal on a liquid crystaldisplay panel and more specifically to a transmission type liquidcrystal display device of the direct viewing type or projection type,which has a light source and can dynamically adjust the luminance of thelight source in accordance with a picture signal to be displayed.

BACKGROUND OF THE INVENTION

[0002] A liquid crystal display device represents a transmission typedisplay device having a light source. The transmission type liquidcrystal display device requires a separate light source since a liquidcrystal panel itself does not emit light. A direct-viewing type liquidcrystal display device has a light source called “back light” mountedbehind the liquid crystal panel, which light source is usually afluorescent lamp generally called a cold cathode tube. A projection typeliquid crystal display device called generally a liquid crystalprojector which uses a lamp such as a halogen lamp and metal halide lampto obtain brightness of an image projected on a screen.

[0003]FIG. 15 shows a general construction of a direct-viewing typeliquid crystal display device. In FIG. 15, there is shown a liquidcrystal display portion 201, a backlight 202, a backlight controlportion 203, a display control portion 204 and an input 205. A picturesignal to be displayed on a liquid crystal display portion 201 is inputin the form of a signal YPbPr (a luminance signal and a color differencesignal) from the input 205. The display control portion 204 performscontrol operations for displaying the input picture signal on the liquidcrystal display portion 201. In practice, the display control portion204 performs, for example, conversion of the signal YPbPr into a signalRGB, rearrangement of picture signals in accordance with the method ofdriving the liquid crystal display portion 201 and adaptive gammacorrection for the liquid crystal display portion 201. The backlight 202is a light source from which the liquid crystal display portion 201obtains luminance. The backlight control portion 203 drives thebacklight and controls the operation of the backlight so that it emitslight always at a specified level of brightness. This backlight controlportion 203 may have a dimming function, which has a plurality of presetvoltage values (or current values) to obtain corresponding predeterminedvalues of brightness of the backlight and regulates the brightness ofthe backlight 202 by selecting the voltage value according to a commandsignal given for example from a personal computer.

[0004]FIGS. 16A and 16B show the relationship between an average levelof brightness of a picture signal to be displayed and a screenbrightness of an image display device. Two kinds of the screenbrightness are preset as brightness control values. Characteristic 16-1relates to the brightness of the backlight, which is obtainable when auser-viewer selects ┌bright┘ mode. It shows that the display screenbrightness 420 candelas is obtainable when a white signal is displayedon the screen. Characteristic 16-2 relates to the brightness of thebacklight, which is obtainable when the user selects ┌normal┘ mode. Itshows that the display screen brightness 260 candelas being isobtainable when a white signal is displayed thereon. As apparent fromFIGS. 16A and 16B, the user-viewer can change, by using the dimmingfunction, the current brightness of the display screen to the otherbrightness which value is always constant independent of a picturesignal to be displayed thereon.

[0005] The display screen luminance (brightness) of the display deviceis determined as a product of multiplication of the transmittance of theliquid crystal display portion 201 by the brightness of light emittedfrom the backlight 202. As described above, since the brightness of thebacklight 202 is constant independently of picture signals, thegradation of each picture signal to be displayed depends upon only thetransmittance of the liquid crystal display portion 201. In other words,the display capability of the image display device can be determined bythe dynamic range of the liquid crystal display portion 201 (i.e., theability to display a white signal and a black signal respectively).

[0006] In recent years, there have been proposed a number of methods forimproving the image quality and the visibility of liquid crystal displaydevices by dynamically regulating the contrast of picture signals andthe brightness of the light source in accordance with picture signalsvarying with time.

[0007] Patent document 1 discloses an example of conventional improvingmethod by dynamically regulating a backlight, which is described belowwith reference to FIG. 17. This method is featured by using an averagebrightness detecting circuit 206 and a backlight control portion 207shown in FIG. 17. An average brightness detecting circuit 206 detects anaverage brightness level of a picture signal. With a high averagebrightness level detected by the circuit 206, the backlight controlportion 207 controls the backlight 202 to decrease its luminance of thelight. The relationship between the average brightness of the picturesignal and the brightness of the display screen is shown in FIG. 18. Asthe brightness of the display screen is thus regulated in accordancewith the average brightness level of a picture signal, it is possible toeffectively prevent the display screen from having excessive brightnessor darkness, thereby providing a display image that the user can viewwith ease and pleasure. This feature enables the user to visually sensean apparently widened dynamic range of the display in comparison withthe case of merely maintaining a constant brightness of the backlight.In other words, the contrast of the image is increased on a dark screenand a bright screen respectively. Furthermore, if an image contains abright part such as a metallic element on a dark background, it can havea high quality with increased luster provided by the effect of theincreased brightness of the metallic part.

[0008] The patent document 2 discloses a conventional improvement of thedisplay image quality of a display device by dynamically regulating boththe contrast of a picture signal and the brightness of a backlight in acertain correlation. The method disclosed in the document 2 increases adynamic range of a picture signal according to average brightness andshifts a level of the picture signal according to an offset value. Thismay cause the level shift of a visually sensible brightness of the imageon the screen, so the backlight brightness is at the same time regulatedby dimming control to absorb the possible level shift of visuallysensible brightness. The visual contrast of an image can be improved bythe above processing.

[0009] Patent document 3 discloses another example of a conventionalmethod of improving the quality of an image by dynamical and correlativecontrol of both the contrast of a picture signal (amplitude modulationof the signal) and the brightness of the backlight (output modulation ofthe light source). The conventional method of the document 3 is suchthat a dark level of a picture signal is detected and, if the durationof the detected dark level exceeds a threshold value, the output levelof the light source is reduced and, at the same time, the dynamic rangeof the picture signal is enlarged. If the dark level duration does notexceed the threshold value, no modulation is done for both the outputlight and the picture signal. The above regulation can reduce theunevenness of the dark-level image area with no affection on thebright-level image area.

[0010] However, the method of the document 1 for improving the displayimage quality by increasing the brightness of the backlight when theaverage brightness level of a picture signal is low involves thefollowing problem. Namely, the liquid crystal display portion 201 mayhave a leakage of light from the backlight even when displaying a blacksignal thereon. The light leakage causes a phenomenon called “loss oftrue black” which makes the display image generally whitish,considerably degrading the image displayed. Therefore, an increase inthe intensity of the backlight is accompanied by losing the true blackimage. The brightness of the light source can not always improve thecontrast of the display image.

[0011] The method of the document 2 for improving the display imagequality by increasing a dynamic range of a picture signal also involvesthe following problem. Namely, if the dynamic range of a picture signalcontaining a picture of collapsed gradation is simply widened, thedefective parts of the picture are emphasized and recognized as defectsby the user. If the dynamic range of a picture signal with a noisecomponent superposed thereon is increased, the noise component is alsoemphasized and recognized as a defect by the user.

[0012] The method of the document 3 for improving the display imagequality by detecting the duration of the black level of the picturesignal and simultaneously modulating both the picture signal and thebacklight output also involves the following problem. Namely, themodulation of both the picture signal and the backlight output isconducted only based on information of the black level of the picturesignal and without referring to other kinds of information such as (forexample average brightness of) the picture signal, so the modulation isnot made for the picture signal if it has no black level, i.e., thedisplay image cannot be improved. Like the method of the document 2, theamplitude modulation of the picture signal emphasizes the defectivelygradated parts and the noise component superposed on the picture signal.

[0013] (Patent Document 1)

[0014] Japanese Laid-Open Patent Publication No. 8-201812

[0015] (Title: Liquid Crystal Display Device)

[0016] (Patent Document 2)

[0017] Japanese Laid-Open Patent Publication No. 2001-27890

[0018] (Title: Image Display Device and Image Display Method)

[0019] (Patent Document 3)

[0020] Japanese Laid-Open Patent Publication No. 6-102484

[0021] (Title: Image Display Device using a spatial light modulatingelement and Image Display Method using the same)

SUMMARY OF THE INVENTION

[0022] An object of the present invention is to provide an image displaydevice capable of displaying high quality video featuring richness inluster and optimal-to-viewer brightness, which device can improvevisually sensible contrast and reproducibility of an image withoutwidening a dynamic range of the picture signal not to cause artifactsand can optimally regulating the display screen brightness not to causethe loss of the true black image.

[0023] The above described object is achieved by the display devicewhich is provided with the following technical means.

[0024] The basic construction of the image display device according tothe present invention comprises an image displaying means for presentingan image, a display control means for performing the control ofoperations for displaying an image on the image displaying means, alight source for illuminating the image displaying means, a light sourcecontrol means for dynamically controlling the brightness of the lightsource, an average brightness detecting means and a peak detecting meansfor detecting a peak of a picture signal.

[0025] The light source control means dynamically controls thebrightness of the light source based on the average brightness of apicture to be displayed on the image displaying means. For displaying apicture signal having low average brightness, the brightness of thelight source is regulated so that the display screen brightness of theimage displaying means can attain a larger value in comparison with thecase when displaying a picture signal having high average brightness.

[0026] The dynamic control of the light source brightness is furthercorrected based on a result of peaks detected by the peak detectingmeans. If a picture signal has a peak satisfying a certain condition ora large number of peaks, the light source brightness is regulated sothat the screen brightness of the image displaying means can attain ahigher value in comparison with the case that a picture signal has nopeak or a small number of peaks. If a picture signal has no peak or asmall number of peaks, the light source brightness is regulated so thatthe screen brightness of the image displaying means can attain a lowervalue in comparison with the case that a picture signal has a peak or alarge number of peaks.

[0027] The dynamic control of the light source brightness is performedby the light source control means by using a characteristic changingpoint corresponding to a certain value of average brightness. In case ofdisplaying a picture signal which average brightness detected by theaverage brightness detecting means is not more than the characteristicchanging point, the brightness of the light source is regulated bychanging the characteristic in advance based on the detected peak asdescribed above. When the picture signal has average brightness beingnot less than the characteristic changing point, then the light sourcebrightness is regulated without changing the characteristic based on thedetected peak.

[0028] Namely, the image display device according to the presentinvention performs the conventional control of the brightness of thelight source and further performs the dynamic control of the brightnessof the same light source based on the detected peak condition of apicture.

[0029] In fact, if the brightness of the light source is increased for apicture signal of low average brightness representing a generally darkpicture having no peak or a small number of peaks, merely the brightnessof black components of the image displaying means is increased,resulting in the loss of true black picture portions. Therefore, thebrightness of the light source is corrected to be reduced for agenerally dark picture, i.e., a picture having low average brightnessand including no peak or a small peak. This is needed to prevent theoccurrence of the loss of the true black picture portions.

[0030] On the contrary, a picture having low average brightness (i.e.,representing a generally dark picture) having a peak or a large numberof peaks is preferably emphasized at its peak portion(s) with increasedbrightness so that it can produce a high contrast and well-modulatedimage on the display screen. In this case, the brightness of the lightsource is regulated to increase. A typical example is an image showing ametal-made finger ring on a dark background, which shall be displayed onthe display screen sufficiently illuminated by the light source to makethe subject stand out with luster of metal on the dark background.

[0031] According to the present invention, no processing for widening adynamic range of a picture signal is made, thereby eliminating thepossibility of emphasizing defective picture components such as adefectively gradated portion and a noise of the picture signal.

[0032] Furthermore, according to the present invention, sinceinformation about a picture signal containing no peak or peaks andrepresenting a dark or bright scene can be obtained in advance from apeak value and average brightness value detected by the peak detectingmeans and the average brightness detecting means respectively, it isalso possible to improve the dynamic contrast of picture by the effectof alternating a dark picture (of low average brightness) with a brightpicture (of high average brightness).

[0033] Thus, the present invention makes it possible to present a highquality display image best adapted to users-viewers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a block diagram showing the construction of an imagedisplay device according to the first embodiment of the presentinvention.

[0035]FIG. 2 is a construction block diagram of a peak detecting portionof the image display device according to the first embodiment of thepresent invention.

[0036]FIGS. 3A and 3B show an example of control characteristics of abacklight control portion of the image display device according to thefirst embodiment of the present invention.

[0037]FIGS. 4A and 4B show another example of control characteristics ofa backlight control portion of the image display device according to thefirst embodiment of the present invention.

[0038]FIGS. 5A and 5B show a further example of control characteristicsof a backlight control portion of the image display device according tothe first embodiment of the present invention.

[0039]FIG. 6 is a construction block diagram of a peak detecting portionof the image display device according to the second embodiment of thepresent invention.

[0040]FIGS. 7A and 7B show an example of control characteristics of abacklight control portion of the image display device according to thesecond embodiment of the present invention.

[0041]FIG. 8 is a construction block diagram of a peak detecting portionof the image display device according to the third embodiment of thepresent invention.

[0042]FIG. 9 is a construction block diagram of a peak detecting portionof the image display device according to the fourth embodiment of thepresent invention.

[0043]FIGS. 10A and 10B show an example of control characteristics of abacklight control portion of the image display device according to thefifth embodiment of the present invention.

[0044]FIG. 11 is a block diagram showing the construction of an imagedisplay device according to the sixth embodiment of the presentinvention.

[0045]FIGS. 12A and 12B show an example of control characteristics of abacklight control portion of the image display device according to thesixth embodiment of the present invention.

[0046]FIG. 13 is a block diagram showing the construction of an imagedisplay device according to the seventh embodiment of the presentinvention.

[0047]FIGS. 14A and 14B show an example of control characteristics of abacklight control portion of the image display device according to theseventh embodiment of the present invention.

[0048]FIG. 15 is a block diagram showing the construction of an imagedisplay device according to a prior art.

[0049]FIGS. 16A and 16B show an example of control characteristics of abacklight control portion of the image display device of FIG. 15.

[0050]FIG. 17 is a block diagram showing the construction of an imagedisplay device according to another prior art.

[0051]FIG. 18 shows an example of control characteristics of a backlightcontrol portion of the image display device of FIG. 17.

PREFERRED EMBODIMENTS OF THE INVENTION

[0052] Preferred embodiments of the present invention will be describedbelow with reference to accompanying drawings.

[0053] (The First Embodiment)

[0054] Referring to FIGS. 1 to 5, an image display device according thefirst embodiment of the present invention is described as follows.

[0055]FIG. 1 is a construction block diagram of the image display deviceaccording to the first embodiment of the present invention, whichcomprises, as shown in FIG. 1, a liquid crystal display portion 11, abacklight 12, a backlight control portion 13, a display control portion14, average brightness detecting portion 15, a peak detecting portion 16and an input 17.

[0056] A PIC (Picture) signal in the form of, for example, a Y colordifference signal to be displayed on the liquid crystal display portion11 is inputted into the display device through the input 17. The displaycontrol portion 14 performs the control operation necessary for displaythe PIC signal on the liquid crystal display portion 11 and outputs itas a DRV (Drive) signal to the display portion. The backlight 12 servesa light source for providing the brightness to a display screen of thetransmission type liquid crystal display portion 11.

[0057] The average brightness detecting portion 15 detects averagebrightness of the PIC signal and outputs it as an AVE (Average) signalto the backlight control portion 13. The average brightness is expressedas a value in percents: a display picture of all black over a wholescreen has the average brightness of 0% and a display picture of allwhite has the average brightness of 100%. The backlight control portion13 receives the AVE signal and controls the brightness of the backlight12 according to a CTL (Control) signal. The backlight control portion 13controls the backlight to increase its brightness with a large AVEsignal and reduce the brightness with a small AVE signal.

[0058] The display device according to the present invention is featuredby the provision of the peak detecting portion 16 which detects a peakof a picture signal and outputs a detection PEK (Peak) signal to thebacklight control portion 13.

[0059]FIG. 2 is a detailed block view of the peak detecting portion 16which comprises a comparator 21, a counter 22, discriminator 23, athreshold value (m) generator 24, a threshold value (n) generator 25, avertical start pulse generator “VRESET-S” 26, a vertical end pulsegenerator “VRESET-E” 27, an input 28 and an output 29.

[0060] A PIC signal through the input 28 is input to the comparator 21which compares a brightness signal or brightness component of each ofsuccessively inputted PIC signals with a threshold value m generated bythe threshold value (m) generator 24 and outputs a comparison result CMP(compare) signal to the counter 22. The CMP signal takes a High statewhen the brightness signal level is larger than the threshold value mwhile it takes a Low state when the brightness signal level is smallerthan the threshold value m. The pulse generator VRESET-S 26 generates apulse V-ST (vertical start) signal in accordance with the vertical startphase of the picture signal. The reset pulse generator VRESET-E 27generates a pulse V-END (vertical end) signal in accordance with thevertical end phase of the picture signal. The vertical start andvertical end of the PIC signal are defined for example by rising andtailing, respectively, of a vertical synchronizing signal accompanyingthe PIC signal. The counter 22 performs counting action for a perioddefined by the V-ST signal and the V-END signal. A counter value isreset by the V-ST signal and +1 is count up with the CMP signal of Highstate, i.e., representing that the brightness level of the PIC signalbeing greater than the threshold value m, and the count value is heldwith the CMP signal of Low state. The counter value is latched by theV-END signal and outputted as a count CNT signal to the discriminator 23which in turn compares the CNT signal with the threshold value nproduced by the threshold (n) generator 25 and outputs a resultant PEKsignal through the output 29. The PEK signal takes a High state forexample when the CNT signal has a value greater than the threshold valuen and it takes a Low state when the CNT signal has a value smaller thanthe same threshold value.

[0061] The peak detecting portion 16 conducts the above describedsequential operations for each of pixels composing a picture signal fora period defined by the V-ST signal and V-END signal. If the PIC signalis of VGA resolution, the number of counts amounts about 300000(=640×480) for the period of a sequence of operations. In this case, thePIC signal is assumed as a digital signal of 8 bits. Hence, the PICsignal has, for example, the brightness level of 0 for all black and 255for all white. It is assumed that the threshold m value is 200 and thethreshold n value is 300. The counter 22 is first reset to zero by theV-ST signal. The comparator 21 compares each of brightness levels ofsuccessively input pixels of the PIC signal with the threshold value m(=200). If the brightness level is not less than the value m (=200),then the comparator outputs a signal High to the counter 22 which inthis case counts +1. If the brightness level is not more than the valuem (=200), then the comparator outputs a signal Low to the counter 22which in this case do no count and holds an already counted value. Withthe V-END signal, the counter 22 transfers a count result to thediscriminator 23 which in turn determines by comparison whether thecount result (the number of counts) is greater or smaller than thethreshold value n (=300). In other words, the brightness level values ofrespective pixel signals successively inputted for a period defined byvertical sync signals (start and end) of a complete picture are comparedwith the threshold m, pixels having the level being not less than m arecounted and the count result is determined whether it is not less thanthe threshold n. When the number of the counted pixels is not less thanthe threshold n, it is decided that the picture contains a peak or peaksbecause n pixels among 300000 pixels composing a picture have values notless than the threshold level m. The decision result is then output fromthe discriminator.

[0062] The counting operation by the counter 22 can be sometimesconducted not for every pixel but every d pixels, where d is anyinteger. In this instance, the processing rate (clock speed) may bereduced to 1/d. There is also a picture whose peak can be surelydetected by sampling every line. In this instance, one line means a unitpixel line of a picture to be vertically scanned for displaying a PICsignal on a liquid crystal display portion 11.

[0063] Although the above described case uses the thresholds m=200 andn=300, it is also possible to use the threshold m=255 that means thedetection of a pixel having a maximum amplitude in a picture. It is alsopossible to use the threshold n=1 to decide the picture has a peak incase that only one of pixels exceeds the threshold level m.

[0064] Referring now to FIGS. 3A and 3B, the operation of the backlightcontrol portion 13 is described below. FIGS. 3A and 3B indicate controlcharacteristics of the backlight control portion 13. In FIG. 3A, theaxis of abscissa represents average brightness values (%) detected bythe average brightness detecting portion 15, with indication of 0% forall black display picture and 100% for all white display picture. Theaxis of ordinate represents screen brightness (cd/m²) of the liquidcrystal display portion 11 which is displaying a white signal (8 bitsdigital signal of level 255) and which is illuminated by the backlight12 regulated by the backlight control portion 13. Average brightness of0% means that a black color signal of level 0 is displayed on a wholescreen area of the liquid crystal display portion. Therefore, theexpression “brightness of the screen displaying thereon a white colorsignal of level 255 at average brightness of 0%” is contradictive in thestrict sense but, for simplicity of explanation, average brightness ofsubstantially 0% is expressed as average brightness of 0%. For example,in case of the VGA resolution system, if the white color of level 255 isdisplayed within an area of 20 by 20 pixels of a whole screen showingthe black color of level 0, the average brightness is determined as0.13% and is simply expressed as 0%. FIG. 3B shows data (numericalvalues) of characteristics shown in FIG. 3A. The shown embodiment isfeatured by the fact that it uses two kinds of characteristics ofaverage brightness versus screen brightness. In FIG. 3A, there are showntwo characteristics 3-1 and 3-2 which are switched over from one to theother with a PEK signal representing a detection result obtained by thepeak detecting portion 16.

[0065] In FIG. 3A, average brightness of 40% is used as a point forswitching over the control characteristic of the backlight 12 bydeciding whether the detected average brightness is grater or smallerthan the point (the average bright level exceeds the above point levelor not). In FIG. 3A, the characteristic 3-1 is selected with the PEKsignal being of High state (i.e., the peak detecting portion 16 decidedthat the picture has a peak) and the backlight 12 is regulated so as toobtain the corresponding screen brightness shown by the selectedcharacteristic 3-1. For example, with the picture signal having lowaverage brightness of 20%, the backlight 12 is regulated so as to obtainthe screen brightness of 440 candelas. When the average brightness is70% or higher, the backlight is regulated so as to obtain the screenbrightness 290 candelas. In case if a picture signal has no peak, thecontrol characteristic 3-2 of FIG. 3A is selected by a PEK signal beingin Low state to regulate the backlight 12 so as to obtain the screenbrightness of, e.g., 390 candelas at a low average brightness of 20%. Inother words, the brightness of the backlight 12 is regulated so as toattain a screen brightness 440 candelas for a picture signal having apeak and a screen brightness 390 candelas for a picture signal having nopeak at the same average brightness.

[0066] As is apparent from FIG. 3A, the backlight for displaying apicture signal having no peak is regulated to be lower than that of thebacklight for displaying a picture signal having a peak. In thisinstance, the backlight is also regulated to be higher than that in thecase when the signal has a high average value of brightness.

[0067] The above described control is to obtain higher brightness ofbacklight for a dark picture signal than that for a bright picturesignal when both signals have a peak respectively. A picture showing,for example, a metal-made spoon on a dark background can be improved invisual quality with increased luster of the metal by increasing thescreen brightness of the liquid crystal display portion 11. A pictureshowing, for example, clear sky being bright in average can be improvedin visual quality by reducing the screen brightness. When two picturesthus processed are presented one after the other, an enhanced visualcontrast is created between two scenes a spoon and clear sky. Whendisplaying a picture signal having no peak, i.e., containing no brightsubject or a few bright parts to be presented desirably with enhancedluster, it is no use of increasing the backlight brightness to improvethe visual quality of the screen image. In this case, an increase in thequantity of light from the backlight is stopped and wasted within theliquid crystal display portion 11, causing the loss of true black imageportion of the displayed image. In this instance, the controlcharacteristic 3-2 is selected so as to reduce the brightness incomparison with the characteristic 3-1 as shown in FIG. 3A.

[0068] In comparison with the case when a whole screen is illuminatedevenly at the brightness of 420 candelas by using a conventionalbacklight having a control characteristic 16-1 as shown in FIGS. 16A and16B, the brightness control by using the characteristic of FIG. 3A alsooffers the advantage of saving power consumption.

[0069] Hardware for attaining the characteristics as shown in FIG. 3Amay be for example a lookup table using a ROM or a polygonalapproximating calculator provided on an LSI.

[0070] Although FIG. 3A relates to an example of linearly controllingthe screen brightness relative to average brightness of an input picturesignal, it is also possible to control the screen brightness by usingnon-linear control characteristics as shown in FIGS. 4A and 4B.

[0071] Although an example of average brightness of 40% being used as acharacteristic changing point was described above with reference to FIG.3A, there may be a case of using average brightness of 50% being used asa characteristic changing point as shown FIGS. 4A and 4B. Therefore theabove average brightness is variable at will.

[0072] Although an example of control characteristic of the backlight interms of the screen brightness of 200 and 500 candelas for displayingpicture signals including a peak and having average brightness of 100%and 0% respectively was described above with reference to FIG. 3A, theremay be a case of using control characteristics of the backlight in termsof the screen brightness of 100 and 350 candelas for displaying picturesignals having average brightness of 100% and 0% respectively andincluding a peak as shown FIGS. 5A and 5B. The relationship between theaverage brightness of the picture signal and the screen brightnessdepends upon the characteristic of the liquid crystal display portion 11and the characteristic of the backlight 12. Accordingly, the visualquality of the screen image on the liquid crystal display portion 11 ofthe actually measured characteristic is determined in practice and theoptimal values for the screen brightness in relation with respectivepicture signals are also determined based on the results of measurementsin view of the power consumption of the device. The control of thebacklight brightness is then conducted under the backlight controlportion 13 based on thus obtained optimal control characteristic values.

[0073] The image display device according to the above describedembodiment of the present invention is provided with a liquid crystaldisplay portion, a backlight, a backlight control portion, a displaycontrol portion, an average brightness detecting portion and a peakdetecting portion and is capable of dynamically controlling thebrightness of the backlight in accordance with an average brightnessvalue and a peak of a picture.

[0074] In other words, this embodiment can dynamically control thebrightness of the backlight in accordance with average brightness of apicture and can further correct the control characteristic of thebacklight in accordance with peak value. Namely, in case of displaying adark picture with no peak, (i.e., a picture including peak not more thana certain condition value and having a low average brightness level),increasing the brightness of backlight increases the brightness of blackelements of the displaying means, causing the loss of true black pictureportions. Therefore, in case of displaying a picture having low averagebrightness and no peak or a small number of peaks, the backlight isfurther corrected so as not to increase its brightness. Dynamic contrastof image can be improved by changing scenes from a dark picture (of lowaverage brightness) to a bright picture (of high average brightness) andreverse.

[0075] The display device can present a high-quality display image withluster at brightness optimal to users-viewers, preventing the occurrenceof loss of the true black color in the image on the display screen.

[0076] (The Second Embodiment)

[0077] An image display device according to the second embodiment of thepresent invention will be described below with reference to FIGS. 6, 7Aand 7B.

[0078]FIG. 6 is a block diagram of a peak detecting portion 16 of thedisplay device which is the second embodiment of the present invention,where parts similar to those shown in FIG. 2 are given the samenumerals. In FIG. 6, there are shown comparators 61, 62, 65, 66,counters 63, 64, a discriminator 67, threshold generators 68 to 71 forgenerating threshold values ma, mb, na and nb respectively and an output72.

[0079] A PIC signal through the input 28 is input to the comparators 61and 62 respectively. The comparator 61 compares each of successivelyinputted brightness signals or brightness components of the PIC signalwith the threshold value ma generated by the threshold (ma) generator 68and outputs a comparison result to the counter 63 which in turn countsinputs for a duration defined by V-ST signal and V-END signal andoutputs a count result to the comparator 65. In the comparator 65, theoutput from the counter 63 is compared with the threshold value nagenerated by the threshold (na) generator 70 and a resultant CMP-Asignal is output to the discriminator 67.

[0080] The comparator 62 compares each of successively inputtedbrightness signals or brightness components of the PIC signal with athreshold value mb generated by the threshold (mb) generator 69 andoutputs a comparison result to the counter 64 which in turn countsinputs for a duration defined by V-ST signal and V-END signal andoutputs a count-up result to the comparator 66. In the comparator 66,the output from the counter 64 is compared with the threshold value nbgenerated by the threshold (nb) generator 71 and a resultant CMP-Bsignal is output to the discriminator 67.

[0081] The discriminator 67 determines from the input CMP-A and CMP-Bsignals a MOD signal which represents one of three stages and outputsthe MOD signal through the output 72.

[0082] This embodiment differs from the first embodiment in that it hastwo peak detecting systems having different thresholds and detects apeak level of a picture signal in three stages. The MOD (Mode) signalbased on the peak level and the frequency of occurrence of that peaklevel represents one of three stages: “no peak”, “a small number ofpeaks and a large number of peaks” to be visually sensible. Thedetermined MOD signal is output to the backlight control portion 13.

[0083] For example, it is assumed that ma=160, mb=200 and na=nb=400.When a picture signal PIC in one scene contains 500 pixels of brightnesslevel 230, both the comparators 65 and 66 judge “there is a peak”, sothe discriminator 67 outputs the MOD signal indicating the existence of“a large number of peaks”.

[0084] For example, when an input picture signal PIC contains 500 pixelsof level 180 and 100 pixels of level 230 in one picture, the comparator65 decides that there is no peak and the comparator 66 decides thatthere is a peak. Consequently, the discriminator 67 outputs a MOD signalindicating “there are a small number of peaks”.

[0085] For example, when an input PIC signal contains 500 pixels oflevel 100 and 100 pixels of level 180 in one picture, both thecomparators 65 and 66 decide “there is no peak” and the discriminator 67in this instance outputs a MOD signal indicating “there is no peak”.

[0086]FIGS. 7A and 7B show control characteristics of the backlightcontrol portion 13 of the present invention. In FIG. 7A, the axis ofabscissa shows the average brightness values detected by the averagebrightness detecting portion 15 and the axis of ordinate shows thescreen brightness values of liquid crystal display portion 11illuminated by the backlight 12.

[0087] As is apparent from FIGS. 7A and 7B, the backlight controlportion 13 in this second embodiment has three different characteristicswhich are selectively changed from one to another by corresponding MODsignals prepared based on the peak detection results from the peakdetecting portion 16 as described with reference to FIG. 6. In practice,the characteristics 7-1, 7-2 and 7-3 are selected with the MOD signalsof the states “there are a large number of peaks”, “there is a smallnumber of peaks” and “there is no peak” respectively and the backlightis controlled. As compared with the first embodiment using two peakdetection states, the present embodiment can carry out more dedicatedcontrol.

[0088] Although the second embodiment has been described above by way ofexample by using three peak-detection states and three correspondingcontrol characteristics, it can also have four or more states and usethe same number of corresponding control characteristics.

[0089] As described above, the present embodiment can detect a pluralityof states based on the results of peak detection made by peak detectingportion and use the same number of corresponding control characteristicsto attain more adaptive processing.

[0090] (The Third Embodiment)

[0091] An image display device according to the third embodiment of thepresent invention will be described below with reference to FIG. 8.

[0092]FIG. 8 is a block diagram of a peak detecting portion 16 of theimage display device which is the second embodiment of the presentinvention, where parts similar to those shown in FIG. 2 are given thesame numerals. In FIG. 8, there are shown comparators 81 and 83, a latch82 and a threshold (p) generator 84.

[0093] The shown embodiment is featured by the provision of thecomparator 81 forming a loop to detect the maximum brightness level ofan input PIC signal to be displayed. Specifically, a FBK (Feedback)signal indicating a comparison result is fed back to an input of thecomparator 81. The feedback operation of the comparator 81 is conductedon each of successively inputted pixels of a PIC signal. The PIC signaland the FBK signal levels are compared with each other on thepixel-by-pixel basis and a larger signal is output as an updated FBKsignal.

[0094] At the image start time of the PIC signal when the comparator 81begins the processing, the FBK signal is reset to zero according to aV-ST signal output by the circuit VRESET-S 26. For example, it isassumed that the PIC signal is input in the order of pixels a0, a1, a2,a3 from the image start time of the PIC signal and levels of therespective pixels are a0=50, a1=200, a2=140 and a3=50. Since the FBKsignal is reset to zero at the image start time, the comparator 81compares the first pixel a0 with zero and outputs a FBK signal of 50(a=0). When a next pixel was inputted, the comparator 81 compares a1with a FBK signal feedback thereto, i.e., compares a1 with a0 in fact.As the result of this, the FBK signal level is updated to take 200 (a1).With a next input, the comparator 81 compares a2 with a1 (feedback FBKsignal) and recognizes that a1 is larger than a2. As the result, the FBKsignal level is kept at 200. With a next input, the comparator 81compares a3 with the feedback FBK signal level and recognizes that thefeedback FBK signal level is larger than a3. As the result, the FBKsignal level is kept at 200. The comparison is thus conducted for eachof pixels successively inputted after the vertical start of the picturesignal to select a greater bright level pixel every time. The operationis repeated until the vertical end of the image is given. The highestbrightness level of pixels composing a picture signal is thus detected.

[0095] At timing of the vertical end of the picture signal, the latch 82takes the operation result of the comparator 81 and outputs the same tothe comparator 83 which in turn compares the output of the latch 82 witha threshold received from the threshold (p) generator 84 to determinewhether the output is larger than the threshold p or not. When theoutput level is higher than the threshold, the comparator 83 decidesthat “there is a peak” and, then, outputs the result in the form of aPEK signal through the output 29 to the backlight control portion 13.Based on the PEK signal from the comparator 83, the backlight controlportion 13 selects suitable one of its control characteristics of FIGS.3A and 3B and regulates the brightness of the backlight 12 according tothe selected control characteristic.

[0096] As described above, the present embodiment can detect the maximumlevel of a picture signal to be displayed on a display screen bycomparing on the pixel-by-pixel basis a current input with a past inputfed back through a comparator loop circuit. The comparison is conductedfor each of pixels composing a picture signal successively inputted fora period from a vertical start to a vertical end of the picture signal.As compared with the first embodiment, this embodiment has a morecompact configuration of its peak detection circuit with no use of acounter and can save the power consumption. Furthermore, the peakdetection system uses only one threshold and can be easily adjusted.

[0097] (The Fourth Embodiment)

[0098] Referring now to FIG. 9, an image display device according to thefourth embodiment of the present invention is described below.

[0099]FIG. 9 is a block diagram depicting the construction of a peakdetecting portion 16 of the image display device according to the fourthembodiment of the present invention. In FIG. 9, parts similar to thoseshown in FIGS. 2, 6 and 8 are given the same numerals. In FIG. 9, thereare shown comparators 91 and 92, a discriminator 93, thresholdgenerators 94 and 95 for generating threshold values (pa) and (pb)respectively and an output 96.

[0100] This embodiment is featured by the fact that each of pixel valuesoutputted from the latch 82 is compared with two kinds of thresholds paand pb to determine the pixel value in one of three kinds of peak statesrepresented by three kinds of PMOD (Peak Mode) signals indicative of “asmall peak”, “a medium peak” and “a large peak” respectively and thecorresponding information is outputted through the output 96 to thebacklight control portion 13.

[0101] For example, it is assumed that pa=200 and pb=150. If an outputof the latch 82 is of the level 230, both the comparators 91 and 92judge “there is a peak”. The discriminator 93 in this instance decidesthat “the peak is large”.

[0102] In the case that, for example, pa=200, pb=150 and the output ofthe latch 82 is of the level 160, the comparator 91 outputs a signalindicating “there is no peak” and the comparator 92 outputs a signalindicating “there is a peak”. In this instance, the discriminator 93decides that “the peak is intermediate”.

[0103] In the case that, for example, pa=200, pb=150 and the output ofthe latch 82 is of the level 100. Both of the comparators 91, 92 outputa signal indicating “there is no peak”. In this instance, thediscriminator 93 decides that “the peak is small”.

[0104] Based on the PMOD signal received from the peak detectingportion, the backlight control portion 13 selects suitable one ofthree-staged control characteristics shown in FIGS. 7A and 7B to controlthe backlight. Characteristic 7-1 is selected with a PMOD signalindicating “a large peak”, characteristic 7-2 is selected with a PMODsignal indicating “an intermediate peak” and characteristic 7-3 isselected with a PMOD signal indicating “a small peak”.

[0105] Although the present embodiment has been described above by wayof example by detecting a peak level in one of three kinds of states andselectively use three corresponding control characteristics, it can alsohave four or more states and use the same number of correspondingcontrol characteristics.

[0106] As described above, the present embodiment can detect a peaklevel in a plurality of states by using a plurality of thresholds asdescribed before for the third embodiment and controls the backlight byselectively using the same number of corresponding controlcharacteristics to achieve more adaptive image processing.

[0107] (The Fifth Embodiment)

[0108] An image display device according to the fifth embodiment of thepresent invention will be described below with reference to FIGS. 10Aand 10B.

[0109]FIGS. 10A and 10B show control characteristics of the backlightcontrol portion 13. In FIG. 10A, the axis of abscissa shows the averagebrightness (%) detected by the average brightness detecting portion 15and the axis of ordinate shows the screen brightness of liquid crystaldisplay portion 11 illuminated by the backlight 12.

[0110] This embodiment is featured by controlling the brightness of thedisplay screen at a constant level in a certain range by usingthresholds instead of average brightness. In FIGS. 10A and 10B, controldata relates to the case that a threshold ra for deciding the low levelof average brightness is 20%. When a detected level of averagebrightness was not more than 20%, the display screen brightness isregulated, independently of the detected average value of the picturesignal, to a constant value which is 500 candelas with a peak detectedby the peak detecting portion 16 and 400 candelas with no peak detectedby the same portion. A threshold rb used for deciding the highbrightness level is 80%. If the detected average brightness was not lessthan 80%, the display screen brightness is regulated, independently ofthe detected average brightness of the picture signal, to a constantvalue which is 200 candelas without consideration of the result of peakdetection by the peak detecting portion 16.

[0111] This feature is made with view of the fact that the frequency ofoccurrence of brightness detection result being not more than thethreshold ra and being not less than the threshold rb is very small. Inthe ranges of average brightness values, which have the low frequency ofoccurrence, fixed values are selected so as to effectively use a dynamicrange of the backlight in the range of the high frequency of occurrence.

[0112] Although the thresholds ra and rb are assumed to be of 20% and80% for the example of characteristics of FIGS. 10A and 10B, they (raand rb) may be set to 30% and 70% respectively. Furthermore, it is alsopossible to use only the threshold ra (preset for example to 30%) andmake no use of the threshold rb by presetting it to 100%. Alternately,it is possible to use only the threshold rb (preset for example to 70%)and make no use of the threshold ra by presetting it to 0%.

[0113] As describe above, the present embodiment is featured by the factthat the backlight control characteristics are given thresholds fordefining brightness ranges in which the display screen brightness isfixed to a certain level independent of the average brightness of apicture signal. The control characteristics may be provided withthresholds on both higher and lower sides of average brightness or athreshold on only one side. This feature offers the advantage ofeffectively using a dynamic range of the backlight controlcharacteristics for the high frequency of occurrence of brightnessdetection results, thereby making it possible for the backlight to beregulated optimally for an image to be displayed on the display screen.

[0114] (The Sixth Embodiment)

[0115] An image display device according to the sixth embodiment of thepresent invention will be described below with reference to FIGS. 11,12A and 12B.

[0116]FIG. 11 is a block diagram showing the construction of aprojection type liquid crystal display device which is the sixthembodiment of the present invention. In FIG. 11, parts similar to thoseshown in FIG. 1 are given the same numerals. In FIG. 11, there is showna screen 111, projection lens 112, a liquid crystal light valve 113, alamp 114 and a lamp control portion 115.

[0117] The lamp control portion 115 controls the lamp 114 based on anAVE signal indicating an average brightness value of a picture signal,which is detected by an average brightness detecting portion 15, and aPEK signal indicating a peak level of the picture signal, which isdetected by a peak detecting portion 16. According to the control of thelamp control portion 115, the lamp 114 illuminates the liquid crystallight valve 113. The liquid crystal light valve 113 is composed of oneor three sheets of transmission type liquid crystal display panels formodulating the light from the lamp (light source) 114 into red, greenand blue components. The light passed through the liquid crystal lightvalve 113 is enlarged by the projection lens 112 and projected onto thescreen 111 to form an image thereon.

[0118]FIGS. 12A and 12B show characteristics of the lamp 114 to becontrolled by the lamp control portion 115. In FIG. 12A, the axis ofabscissa shows the average brightness (%) detected by the averagebrightness detecting portion 15 and the axis of ordinate shows thescreen brightness (lumen) of the screen 111 illuminated by the lamp 114under the control of the lamp control portion 115. Controlcharacteristics 12-1 and 12-2 are changed over one to the other by a PEKsignal. Namely, when the PEK signal from the peak detecting portion 16indicates there is a peak or there are may peaks, the lamp 114 isregulated by the lamp control portion 115 so that the screen brightnessdesignated by the control characteristic 12-1 may be obtained. With aPEK signal indicating there is no-peak, the lamp 114 is regulated sothat the screen brightness designated by the control characteristic 12-2may be obtained.

[0119] As described above, the image display device according to thepresent invention can be used not only as a direct viewing type liquidcrystal display device but also as a projection type liquid crystaldisplay device.

[0120] (The Seventh Embodiment)

[0121] An image display device according to the seventh embodiment ofthe present invention will be described below with reference to FIGS.13, 14A and 14B.

[0122]FIG. 13 is a block diagram showing the construction of an imagedisplay device which is the seventh embodiment of the present invention,where parts similar to those shown in FIG. 1 are given the same numeralsand characters.

[0123] In FIG. 13, numeral 101 designates an image discriminator whichdetermines the feature of an image to be displayed. The discriminationresult is output as a P-BRT (Picture-Bright) signal and a P-PEK(Picture-Peak) signal to a backlight control portion 13. The P-BRTsignal indicates a degree of brightness of a picture to be displayed,which degree is determined for example as a result of detection ofaverage brightness of the picture. The P-PEK signal indicates a degreeof a peak value of the picture to be displayed, which can be obtainedfor example by the processing as described in the first embodiment withreference to FIG. 2.

[0124]FIGS. 14A and 14B show control characteristics of the backlightcontrol portion 13 of the image display device according to the seventhembodiment of the present invention. In FIG. 14A, the axis of abscissarepresents the P-BRT signal showing that a larger signal valuerepresents a brighter picture to be displayed or a greater averagebrightness of the same picture. The axis of ordinate shows thebrightness (luminance) of the light-emitting surface of the backlight12. For example, when the transmittance of the liquid crystal displayportion 11 is 5% and the brightness of the backlight is 2000 candelas,the brightness of the display image formed by the light passed throughthe liquid crystal display portion 11 is about 100 candelas.

[0125]FIGS. 14A and 14B describe two kinds of control characteristics14-1 and 14-2 which are selected by the backlight control portionaccording to the P-PEK signal. Specifically, the characteristic 14-2 isselected to use for controlling the backlight when a received P-PEKsignal has a small value and the characteristic 14-1 is selected when areceived P-PEK signal has a large value. In other words, FIGS. 14A and14B indicate that the backlight is regulated to increase the lightquantity when a picture signal has a large peak or a large number ofpeaks and a display screen is dark and it is regulated to decrease thelight quantity when a picture signal has a small peak or a small numberof peaks and a display screen is dark.

[0126] As described above, this embodiment of the present invention canregulate the quantity of light emitted from the backlight (light source)by using a means for detecting whether a display screen is darker than aspecified brightness level or a picture to be displayed is dark orbright and the peak detecting means for detecting a peak of a picture.The control characteristic is such that when a picture has an averagebrightness level lower than a specified value and has a peak or a largenumber of peaks, the light source must be regulated to have a greaterbrightness value in comparison with the case when a picture has no peakor a small number of peaks. By doing so, when displaying a sequence ofscenes of fireworks rising in a night sky, only a scene showing a brightshot of fireworks appears in the dark night sky is displayed on a screenwith an increased quantity of light from the backlight so as to increasethe luster of sparks of the fireworks busted in the dark sky. The burstof the fireworks can be detected by using the peak detection function.The above processing makes it possible to sufficiently present thebrilliance of the fireworks and to create an enhanced dynamic contrastof the visual presentation by the effect of changing a scene of thefireworks (in the night sky) with a generally bright scene (spectators).

[0127] As is apparent from the foregoing, an image display deviceaccording to the present invention is provided with an image displayingmeans composed of a direct-viewing type liquid-crystal display or aprojection type liquid-crystal display, a display control means, a lightsource such as a backlight, a light source control means such as abacklight control means, an average brightness detecting means and apeak detecting means and can dynamically control the brightness of thelight source (backlight) in accordance with the average brightness andthe peak level of a picture to be displayed on the image displayingmeans.

[0128] According to the present invention, it is possible to dynamicallycontrol the backlight according to the average brightness of a picturesignal to be displayed. Namely, the brightness of the backlight isdecreased for displaying a bright picture having high average brightnesswhile it is increased for displaying a dark picture having small averagebrightness. In comparison with a conventional image display device witha backlight emitting light having a fixed brightness value, which maypresent an excessively bright or dark picture unpleasant to theuser-viewer, the image display device of the present invention can offerthe advantage of presenting to users-viewers a pleasant picture byincreasing or reducing the backlight brightness for a dark picture or abright picture. According to the present invention, it is also possibleto enhance the visual contrast of image by alternating a bright picturewith a dark picture.

[0129] According to the present invention, it is also possible todynamically correct for the control characteristic of the backlight(light source) in accordance with a peak value of a picture. Namely, ifthe backlight brightness was increased for displaying a dark picturewith no peak, (i.e., peak of a picture to be displayed not more than acertain condition value and having a low average brightness level), itmay increase the brightness of black elements of the image displayingmeans causing the loss of true black picture portions. Therefore, whendisplaying a picture having a low average brightness value and having nopeak or a small number of peaks, the brightness of the backlight isdynamically corrected not to increase. This makes it possible to presenta picture with increased luster avoiding the occurrence of loss of trueblack.

[0130] Furthermore, the image display device according to the presentinvention can increase the brightness of the backlight when presenting agenerally dark picture having a peak or large number of peaks. Forexample, a picture showing a metal-made finger ring on a dark backgroundis displayed on a screen illuminated sufficiently by increasing thebrightness of the backlight. The picture thus reproduced may show thelustrous ring standing out on the background with no loss of true blackportion thereof.

[0131] Since the image display device according to the present inventiondoes not perform any processing for widening a dynamic range of apicture signal, thereby eliminating the possibility of causing collapseof gradation and/or enhancement of noise components.

[0132] Generally speaking, the image display devices according to thepresent invention can present an image optimally adapted to theuser-viewer.

1-62. (Cancelled).
 63. A liquid crystal display device capable ofcontrolling brightness of a light source in accordance with averagebrightness of an input picture, wherein the device is provided with apeak detecting means for detecting a degree of peak level of the inputpicture in a plurality of stages and increases the brightness of thelight source as the input picture has a larger degree of peak level. 64.A liquid crystal display device as defined in claim 63, wherein, in acase when the average brightness of an input picture is not more than apreset value, the device increases the brightness of the light source asthe input picture has a larger degree of peak level.
 65. A liquidcrystal display device capable of controlling brightness of a lightsource in accordance with average brightness of an input picture,wherein the device is provided with a peak detecting means for detectinga degree of peak level of the input picture in a plurality of stages anddecreases the brightness of the light source as the input picture has asmaller degree of peak level.
 66. A liquid crystal display device asdefined in claim 65, wherein, in a case when the average brightness ofan input picture is not more than a preset value, the device decreasesthe brightness of the light source as the input picture has a smallerdegree of peak level.
 67. A liquid crystal display device as defined inany one of claims 63 to 66, wherein the device prevents the brightnessof the light source from increasing as an input picture has a largeraverage brightness.
 68. A liquid crystal display device capable ofcontrolling brightness of a light source in accordance with averagebrightness of an input picture, wherein the device is provided with apeak detecting means for detecting a degree of peak level of the inputpicture in a plurality of stages and corrects a control characteristicof the brightness of the light source in relation to the averagebrightness of the input picture so as to increase the brightness of thelight source as the input picture has a larger degree of peak level. 69.A liquid crystal display device as defined in claim 68, wherein, in acase when the average brightness of an input picture is not more than apreset value, the device corrects a control characteristic of thebrightness of the light source in relation to the average brightness ofthe input picture so as to increase the brightness of the light sourceas the input picture has a larger degree of peak level.
 70. A liquidcrystal display device capable of controlling brightness of a lightsource in accordance with average brightness of an input picture,wherein the device is provided with a peak detecting means for detectinga degree of peak level of the input picture in a plurality of stages andcorrects a control characteristic of the brightness of the light sourcein relation to the average brightness of the input picture so as todecrease the brightness of the light source as the input picture has asmaller degree of peak level.
 71. A liquid crystal display device asdefined in claim 70, wherein, in a case when the average brightness ofan input picture is not more than a preset value, the device corrects acontrol characteristic of the brightness of the light source in relationto the average brightness of the input picture so as to decrease thebrightness of the light source as the input picture has a smaller degreeof peak level.
 72. A liquid crystal display device as defined in any oneof claims 68 to 71, wherein the device further prevents the brightnessof the light source from increasing as an input picture has a largeraverage brightness.
 73. A liquid crystal display device as defined inany one of claims 63, 64, 65, 66, 68, 69, 70, and 71, wherein the peakdetecting means decides the degree of peak level of an input picture asbeing large in a case when the input picture contains n or more pixelseach being larger than a level m (where n and m are preset values). 74.A liquid crystal display device as defined in any one of claims 63, 64,65, 66, 68, 69, 70 and 71, wherein the peak detecting means detects acase that an input picture contains na or more pixels each being largerthan a level ma (where na and ma are preset values) and a case that theinput picture contains nb or more pixels each being larger than a levelmb (where nb and mb are preset values) and decides a degree of peaklevel by stages.
 75. A liquid crystal display device as defined in anyone of claims 63, 64, 65, 66, 68, 69, 70 and 71, wherein the peakdetecting means has at least three combinations of threshold level mxand threshold number nx for detecting a case that an input picturecontains nx or more pixels each being larger than a level mx (where nxand mx are preset values) and decides the degree of peak level by stagesbased on results of comparison of the combinations.
 76. A liquid crystaldisplay device as defined in any one of claims 63, 64, 65, 66, 68, 69,70 and 71, wherein the peak detecting means detects a maximum levelwithin an input picture, compares the maximum level with a level p(where p is a preset value) and decides a degree of peak level as beinglarge when the maximum level is larger than the level p.
 77. A liquidcrystal display device as defined in any one of claims 63, 64, 65, 66,68, 69, 70 and 71, wherein the peak detecting means detects a maximumlevel within an input picture, compares the maximum level with a levelpa and a level pb respectively (where pa and pb are preset values) anddecides the degree of peak level by stages.
 78. A liquid crystal displaydevice as defined in any one of claims 63, 64, 65, 66, 68, 69, 70 and71, wherein the peak detecting means detects a maximum level within aninput picture, compares the maximum level with at least three levels px(where px is a preset value) and decides the degree of peak level basedon results of the respective comparison.
 79. A liquid crystal displaydevice as defined in any one of claims 64, 66, 69, 70 and 71, whereinthe device has a first threshold value ra (where ra is a preset value)being smaller than the preset value and controls the light source tohave a fixed value of brightness in a case when the average brightnessof an input picture is smaller than the first threshold value ra.
 80. Aliquid crystal display device as defined in any one of claims 64, 66,69, 70 and 71, wherein the device has a second threshold value rb (whererb is a preset value) being larger than the preset value and controlsthe light source to have a fixed value of brightness in a case when theaverage brightness of the input picture is larger than the secondthreshold value rb.
 81. A display control method for controllingbrightness of a light source in accordance with average brightness of aninput picture, comprising steps of: detecting a degree of peak level ofthe input picture in a plurality of stages; and increasing thebrightness of the light source as the input picture has a larger degreeof peak level.
 82. A display control method for controlling brightnessof a light source in accordance with average brightness of an inputpicture, comprising steps of: detecting a degree of peak level of theinput picture in a plurality of stages; and decreasing the brightness ofthe light source as the input picture has a smaller degree of peaklevel.
 83. A display control method for controlling brightness of alight source in accordance with average brightness of an input picture,comprising steps of: detecting a degree of peak level of the inputpicture in a plurality of stages; and correcting a controlcharacteristic of the light source in relation to the average brightnessof the input picture so as to increase the light source of thebrightness of the light source as the input picture has a larger degreeof peak level.
 84. A display control method for controlling brightnessof a light source in accordance with average brightness of an inputpicture, comprising steps of: detecting a degree of peak level of theinput picture in a plurality of stages and correcting a controlcharacteristic of the light source in relation to the average brightnessof the input picture so as to decrease the brightness of the lightsource as the input picture has a smaller degree of peak level.
 85. Animage display device capable of controlling brightness of a light sourcein accordance with average brightness of an input picture, wherein thedevice is provided with a peak detecting means for detecting a degree ofpeak level of the input picture in a plurality of stages and correctsthe brightness of the light source in accordance with the degree of peaklevel in a case when the average brightness of the input picture is notmore than a preset value.
 86. An image display device as defined inclaim 85, wherein the device controls the brightness of the light sourceindependently of a degree of peak level in a case when the averagebrightness of the input picture exceeds the preset value.
 87. An imagedisplay device as defined in any one of claims 85 and 86, wherein thedevice controls the brightness of the light source so that thebrightness of the light source in a case when the average brightness ofthe input picture is not more than the preset value may be higher thanthe brightness of the light source in a case when the average brightnessof the input picture exceeds the preset value.
 88. An image displaydevice as defined in any one of claims 85 and 86, wherein the peakdetecting means decides the degree of peak level of an input picture asbeing large in a case when the input picture contains n or more pixelseach being larger than a level m (where n and m are preset values). 89.An image display device as defined in any one of claims 85 and 86,wherein the peak detecting means detects a case that an input picturecontains na or more pixels each being larger than a level ma (where naand ma are preset values) and a case that the input picture contains nbor more pixels each being larger than a level mb (where nb and mb arepreset values) and decides a degree of peak level by stages.
 90. Animage display device as defined in any one of claims 85 and 86, whereinthe peak detecting means has at least three combinations of thresholdlevel mx and threshold number nx for detecting a case that an inputpicture contains nx or more pixels each being larger than a level mx(where nx and mx are preset values) and decides the degree of peak levelby stages based on results of comparison of the combinations.
 91. Animage display device as defined in any one of claims 85 and 86, whereinthe peak detecting means detects a maximum level within an inputpicture, compares the maximum level with a level p (where p is a presetvalue) and decides the degree of peak level as being large when themaximum level is larger than the level p.
 92. An image display device asdefined in any one of claims 85 and 86, wherein the peak detecting meansdetects a maximum level within an input picture, compares the maximumlevel with a level pa and a level pb respectively (where pa and pb arepreset values) and decides the degree of peak level by stages.
 93. Animage display device as defined in any one of claims 85 and 86, whereinthe peak detecting means detects a maximum level within an inputpicture, compares the maximum level with at least three levels px (wherepx is a preset value) and decides the degree of peak level based onresults of the respective comparison.
 94. An image display device asdefined in any one of claims 85 and 86, wherein the device has a firstthreshold value ra (where ra is a preset value) being smaller than thepreset value and controls the light source to have a fixed value ofbrightness in a case when the average brightness of an input picture issmaller than the first threshold value ra.
 95. An image display deviceas defined in any one of claims 85 and 86, wherein the device has asecond threshold value rb (where rb is a preset value) being larger thanthe preset value and controls the light source to have a fixed value ofbrightness in a case when the average brightness of the input picture islarger than the second threshold value rb.
 96. A display control methodfor controlling brightness of a light source in accordance with averagebrightness of an input picture, comprising steps of: detecting a degreeof peak level of the input picture in a plurality of stages; andcorrecting brightness of the light source in accordance with the degreeof peak level in a case when the average brightness of the input pictureis not more than a preset value.
 97. A display control method forcontrolling brightness of a light source in accordance with averagebrightness of an input picture, comprising steps of: detecting a degreeof peak level of the input picture in a plurality of stages; andcontrolling the brightness of the light source by correcting thebrightness in accordance with the degree of peak level in a case whenthe average brightness of the input picture is not more than a presetvalue and controlling the brightness of the light source independentlyof the degree of peak level when the average brightness of the inputpicture exceeds the preset value.
 98. An image display device capable ofcontrolling brightness of a light source in accordance with averagebrightness of an input picture, comprising a picture display means, alight source, a light source control means and an average brightnessdetecting means, wherein a peak detecting means is provided fordetecting a degree of peak level of the input picture and the lightsource control means has a characteristic changing point correspondingto an average brightness value obtainable as a result of detection bythe average brightness detecting means, controls the light sourceindependently of a result of detection by the peak detecting means whenthe detected average brightness value exceeds the characteristicchanging point and corrects the screen brightness of the image displaymeans by controlling the light source in accordance with the result ofdetection by the peak detecting means when the average brightness valuedoes not exceed the characteristic changing point.
 99. An image displaydevice as defined in claim 98, wherein the picture display means is adirect-viewing type liquid crystal display.
 100. An image display deviceas defined in claim 98, wherein the picture display means is aprojecting type liquid crystal display.